The present invention relates to an ultraviolet (UV)-curable adhesive for bonding optical disks, and more particularly to an ultraviolet-curable adhesive for bonding optical disks, which exhibits a sufficient curability of the surface (light irradiation side) and the inner portion (portion in the vicinity of the interface with an optical disk located on the opposite side to the light irradiation side) even by a slight amount of UV rays, the adhesive layer formed from which shows a uniform cured state and does not exert a bad influence upon a recording film even under high temperature and high humidity conditions, and which does not cause deposition of components even if stored for a long term.
As an ultraviolet-curable adhesive for bonding optical disks are popular those wherein a polymerization initiator is incorporated in a radical-polymerizable vinyl compound. These adhesives are required to be curable in a short period of time by irradiation of light that has passed through a disk having an information recording film and accordingly is low in energy.
As a technique satisfying such a requirement, WO 97/40115 discloses an adhesive that uses an .alpha.-hydroxyalkylphenone compound and an acylphosphine oxide compound as a photopolymerization initiator. Optical disks having a low light transmittance can be easily bonded in a short time by this adhesive without impairing the appearance of the disks. However, the adhesive has the problems that deposition is easy to occur since the melting point of the photopolymerization initiator is high, phosphate ion is generated at the time of reaction and remarkably corrodes the recording film, and the stability under fluorescent lamp is poor and the viscosity is easy to increase. Thus, the proposed adhesive is not suitable for use in bonding optical disks.
Japanese Patent Publication Kokai No. 9-169957 discloses an adhesive that uses an acylphosphine oxide compound and/or an .alpha.-aminoacetophenone compound as a photopolymerization initiator. However, use of the acylphosphine oxide compound causes the same problems as above. When the .alpha.-aminoacetophenone compound is used alone without using the acylphosphine oxide compound, balance in curability between the surface and the depths and the stability under fluorescent lamp are inferior and, since the adhesive is easy to cause yellowing, the appearance of disks is easy to be impaired.
It is an object of the present invention to solve the above-mentioned problems and to provide an ultraviolet-curable adhesive for bonding optical disks which can prepare optical disk products which are small in warping of disks, retain sufficient performances even under high temperature and high humidity and have a good appearance.
In accordance with the present invention, there is provided an ultraviolet-curable adhesive for bonding optical disks, which comprises, as essential components,
(A) a radical-polymerizable vinyl compound,
(B) an .alpha.-hydroxyalkylphenone compound, as a photopolymerization initiator, represented by the formula (1): ##STR1##
wherein R.sup.1 is a hydrogen atom or an aliphatic group having 1 to 10 carbon atoms, R.sup.2 and R.sup.3 are the same or different and each is a hydrogen atom or an alkyl group having 1 to 10 carbon atoms, and R.sup.2 and R.sup.3 may be joined to form a single ring having 4 to 8 carbon atoms, and
(C) 2-methyl-1-[4-(methylthio)phenyl]-2-morpholino-propane-1-one.
Preferably, the total amount of the components (B) and (C) is from 0.1 to 10 parts by weight per 100 parts by weight of the component (A).
It is preferable that the component (A) contains a urethane (meth)acrylate oligomer and a difunctional component. An oligomer derived from a caprolactone-based polyol, an alicyclic diisocyanate and (meth)acrylic acid is preferred as the urethane (meth)acrylate oligomer, and a bisphenol type di(meth)acrylate and/or tetraethylene glycol diacrylate are preferred as the difunctional component.